defined module-level constant for AiryDisk2D model; fixed bug with peak-pixel amplitude
defined module-level constant for AiryDisk2D model; fixed bug with peak-pixel amplitude

DougBurke/astropy,dhomeier/astropy,saimn/astropy,tbabej/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,AustereCuriosity/astropy,kelle/astropy,lpsinger/astropy,funbaker/astropy,dhomeier/astropy,mhvk/astropy,larrybradley/astropy,mhvk/astropy,aleksandr-bakanov/astropy,pllim/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,astropy/astropy,joergdietrich/astropy,stargaser/astropy,mhvk/astropy,joergdietrich/astropy,DougBurke/astropy,tbabej/astropy,StuartLittlefair/astropy,lpsinger/astropy,tbabej/astropy,stargaser/astropy,pllim/astropy,bsipocz/astropy,AustereCuriosity/astropy,tbabej/astropy,AustereCuriosity/astropy,larrybradley/astropy,dhomeier/astropy,astropy/astropy,pllim/astropy,funbaker/astropy,DougBurke/astropy,mhvk/astropy,StuartLittlefair/astropy,MSeifert04/astropy,MSeifert04/astropy,larrybradley/astropy,saimn/astropy,StuartLittlefair/astropy,funbaker/astropy,lpsinger/astropy,saimn/astropy,astropy/astropy,bsipocz/astropy,joergdietrich/astropy,dhomeier/astropy,saimn/astropy,larrybradley/astropy,lpsinger/astropy,kelle/astropy,stargaser/astropy,saimn/astropy,DougBurke/astropy,tbabej/astropy,kelle/astropy,kelle/astropy,kelle/astropy,pllim/astropy,MSeifert04/astropy,MSeifert04/astropy,mhvk/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,joergdietrich/astropy,StuartLittlefair/astropy,lpsinger/astropy,astropy/astropy,aleksandr-bakanov/astropy,pllim/astropy,astropy/astropy,AustereCuriosity/astropy,stargaser/astropy,larrybradley/astropy,funbaker/astropy